Such improvements are described in IT-U-9044, in IT-U-209674 and above all, in the more recent EP-363.938 wherein, to allow eliminating dangerous rebounds and make the operating times more uniform, the electromagnetic unit has two coils, so that the movable core to stop the weft yarn is controlled both to take up its working position and to take up its position of rest.
Though the performances of the electromagnetic stopping unit have thus been improved, the situation is still critical when the weft yarn is being cut by the loom cutting device positioned at the outlet of the main nozzle feeding the weft yarn into the loom shed.
In fact, in this step there is a sudden change of weft yarn tension causing oscillations along the tensioned yarn, which return from its cut end up to the point in which the yarn has been blocked by the rod of the electromagnetic stopping unit, and which may continue up to reaching the turns wound on the weft feeder drum.
Such oscillations, which produce on the yarn strong transversal and longitudinal waves, can cause said yarn to wedge under the rod of the electromagnetic stopping unit--in a working position into the drum hole--even after the yarn has stopped against said rod; they can also cause ruffling of the yarn turns wound in the area of its outlet from the feeder drum, close to said electromagnetic stopping unit.
This behavior obviously determines an incorrect measuring of the weft yarn length being inserted into the loom, producing faults which may have different consequences, but which always involve damages.
In fact, a general consequence is that a yarn turn may end up by sliding under the rod of the electromagnetic stopping unit (in particular cases it could even be more than one turn) after the weft yarn has been inserted into the loom, and this could lead to a successive weft yarn insertion including a yarn length which is longer than one turn--in case of a single-colored weave--or else it could lead to the simultaneous insertion of a one color turn with a different color turn, in case of a many-colored weave.
This drawback is particularly felt in the case of a many-colored weave, in that it can show up when changing of the color takes place, in the form of an insertion corresponding to one or more turns, together with the next color and in the same shed, with no possibility of being detected by the weft yarn control systems used at present on fluid jet looms.
To overcome this drawback the EP-239.055, filed by the Applicant, supplied a device to be applied at the outlet of measuring weft feeders, apt to greatly dampen the oscillating and vibratory motion of the weft yarn at the outlet of said feeder, particularly upon cutting of the yarn.
Said solution, making use of an additional device, resulted however uneconomic and, quite often, not very convenient for the user.
The Applicant then adopted a particular arrangement for the drum hole into which engages the rod of the electromagnetic stopping unit. According to this solution, described in EP-441.288, a plurality of bristles is positioned into said hole, transversally to the rod of the unit, to allow said rod to engage said bristles and move at least partially through them when taking up its projecting position to stop the yarn.
Said arrangement no doubt represents a valid solution to prevent the weft yarn from sliding under the rod of the electromagnetic stopping unit, in that the bristles form a real barrier which blocks the passage of said yarn. It nevertheless still represents an extra device, added to the other parts of the measuring weft feeder and, in particular, to that part of the yarn reserve drum which carries the hole into which engages the rod of the electromagnetic stopping unit; furthermore, said arrangement is anyhow not apt to eliminate the loosening and consequent ruffling and crossing of the yarn turns wound on the drum.
The present invention now proposes to solve the above problem at the root, by suitably improving the electromagnetic stopping unit without using any additional devices.